This invention relates to a method of producing a photomultiplier tube including a vacuum container having an incident window, a photocathode formed on the inner surface of the incident window and an electron multiplier element spaced from the photocathode, and more particularly to a method of producing a photomultiplier tube in which the photocathode having high sensitivity and high uniformity can be easily formed.
In general, in order to form a photocathode excellent in characteristic and in uniformity a depositing source for depositing constituents for forming the photocathode should be spaced from the inner surface of the incident window, and the distance there between should be at least about the diameter of the photocathode. Further, depending on its use, the distance between the photocathode and the electron multiplier element must be decreased.
For instance, the paper entitled "Position Sensitive Photomultiplier Tube for Scientillation Imaging" by Eiji Kume, Shinichi Muramatsu and Masahiro Iida presented at NSS in Nov. 1985 has disclosed a position sensitive photomultiplier tube in which the position of a light beam applied to the photocathode can be obtained.
In such a position sensitive photomultiplier tube, the position of a light beam applied to the photocathode cannot be obtained unless the photocathode is disposed close to the electron multiplier element. Accordingly, unlike a conventional photomultiplier tube, it is impossible to provide the depositing source inside the tube, and therefore the constituents are deposited before the tube is sealed.
However, this method is disadvantageous in that, due to oxygen in the air and heating during the tube sealing operation, the sensitivity of the photocathode is lower than that of the ordinary photomultiplier tube.
On the other hand, in the case of a proximity-type image intensifier, the photocathode is formed in the same vacuum device, and is then combined with the body of the photomultiplier tube provided at a different position, and then the tube is sealed. In this case, the photocathode can be disposed close to the electron multiplier element, and the sensitivity of the photocathode can be made substantially equal to that of the ordinary photomultiplier tube.
However, this method is also disadvantageous in that a manufacturing machine for producing a photomultiplier tube is difficult to handle, and is not suitable for mass production, so that the manufacturing cost is high.